<button id="startOsc">Start Oscillator</button>

<button id="stopOsc">Stop Oscillator</button>

let actx = new AudioContext();
let osc1 = actx.createOscillator();
let gain1 = actx.createGain();
let out = actx.destination;

osc1.connect(gain1);
gain1.connect(out);

const startButton = document.querySelector('button#startOsc');

startButton.addEventListener('click', () => {
  osc1.start();
});

const stopButton = document.querySelector('button#stopOsc');

stopButton.addEventListener('click', () => {
  osc1.stop();
});